1. Field of the Invention
The present invention relates to techniques for vaporizing the eluent from a chromatography for detection or analysis and, more particularly, relates to improved thermospray techniques for vaporizing relatively low-flow rate samples separated by liquid chromatography for high sensitivity analysis by a quadrupole mass spectrometer.
2. Description of the Background
In the field of analytical chemistry, compounds separated by gas chromatography (GC) have long been analyzed by mass spectrometry (MS), since such compounds can be easily altered to an ion vapor suitable for MS analysis. Although gas chromatography is used to separate various mixtures, certain samples can only be effectively separated by liquid chromatography (LC).
Some compounds separated by liquid chromatography can be heated to vaporization and then converted to an ion vapor by bombarding the compound in its gaseous state with a beam of electrons (electron impact ionization or EI), and other compounds separated by LC can be heated to vaporization and subjected to chemically reactive ions (chemical ionization) for MS analysis. Many compounds having large, thermally labile molecules separated by LC are not, however, sufficiently volatile at ambient temperatures to form a suitable gas, and other compounds decompose when subjected to heat. Still other compounds can best be separated by supercritical fluid chromatography (SFC), a process wherein the chromatographic eluent acts much like a liquid rather than a gas.
An increasing popular technique referred to as thermospray has recently become accepted in the field of analytical chemistry for converting the eluent from the liquid chromatograph into an ionized vapor suitable for mass spectrometry analysis. A thermospray LC/MS interface is disclosed in U.S. Pat. No. 4,730,111, and hereby incorporated by reference. Thermospray interfaces are widely used today in more than 200 laboratories throughout the United States and Europe, and provide an effective technique for vaporizing the LC eluent for MS analysis. A principal advantage of this thermospray technique compared to other vaporization techniques is that non-volatile solutes separated by LC can be vaporized for MS analysis without producing uncontrolled pyrolysis of the solute, and without allowing the solute to "salt out" on solid surfaces. Moreover, various large, thermally labile molecules not amenable to conversion into an ion vapor by conventional vaporization techniques can be effectively vaporized by the thermospray techniques for MS analysis. Thus the thermospray vaporization technique is rapidly becoming a preferred technique for converting various LC eluents for MS analysis.
Thermospray may be defined as the controlled, partial vaporization, and in many instances almost but most importantly not the complete vaporization, of a liquid stream as it flows through the capillary tube of the vaporizer. In the LC/MS thermospray vaporization process, a jet of vapor is created normally containing a mist of fine particles or solvent droplets in a jetstream of vaporized gas. By partially vaporizing the solution within the capillary tube, the expanding vapor phase of the solution creates an intense vapor jet expelled at supersonic velocity from the capillary tube.
A controlled combination of a cartridge heater imbedded in a copper block which is in intimate thermal contact with the end of the capillary tube and direct ohmic heating of the tube may be used to obtain the desired constant degree of vaporization. As the droplets travel out of the capillary tube to a temperature and pressure controlled environment, they continue to vaporize due to the addition of heat to this environment from the cartridge heater and from rapid heat input from the surrounding hot vapor. As a result of heating, the liquid is nebulized and partially vaporized and any unvaporized solvent and sample are carried into the ion source as micro droplets or particles in a supersonic jet of vapor. Nonvolatile samples may be ionized by direct ion evaporation from highly charged droplets or particles, and more volatile samples may be ionized directly or by ion-molecule reactions in the gas phase. The thermospray vaporizer thus controls the partial vaporization of the sample in the probe and transits the vaporized ions through its ion exit aperture and to the mass spectrometer.
Although thermospray has significant advantages over prior art vaporization techniques for many applications, present-day thermospray technology has limited acceptance due to poor performance at low LC flow rates. Thermospray techniques are disclosed in the referenced patent have successfully been applied to conventional LC units operating at flows in the 0.5 to 2 ml per minute range, and performance is optimized under reversed-phase chromatography using volatile aqueous buffers. Conventional LC columns in the 2 mm to 5 mm range thus produce sufficient flow rates to be directly connected by existing thermospray techniques to MS. The present-day thermospray techniques are not, however, generally compatible on-line with a capillary LC or small diameter packed column LC which yield much lower flow rates. Considerable progress has recently been made in developing quality LC columns of smaller diameter, although present-day thermospray techniques do not yield a sufficient quantity of ions to the mass spectrometer for such low LC flow rates to produce satisfactory results. One solution for increasing the flow rate for existing thermospray applications is to add a favorable solvent, such as ammonium acetate, to add make-up flow downstream from the LC column. This approach has the disadvantage, however, of diluting the sample, and thus MS sensitivity decreases dramatically at low LC flow rates.
It has been recognized for some time that a reduced diameter nozzle at the end of a vaporizer capillary would allow higher temperatures and velocities at the exit of the vaporizer, while providing lower liquid velocities in the capillary where heat transfer is occurring. Accordingly, some vaporizer users have crimped the end of a vaporizer capillary with a pliers in an attempt to provide an improved vaporized jet stream. Such techniques generally do not, however, yield quality vaporizing performance for various applications, and the pattern of the jet stream is generally adversely affected by this irregular crimping operation. Since commercially available supplies of such tubing are limited in size and each size tends to have a substantially variable diameter along its length, wide variations in the effective exit diameter of the capillary tube produce inconsistent results. Moreover, conventional vaporizer capillary tubes can become plugged as a result of deposition of the non-volatile material, thereby rendering the entire vaporizer unusable for subsequent use.
Additional problems arise with the use of comparatively small diameter capillary tubes in vaporizers, particularly with respect to heat transfer and pressure drop. In tubes with similar characterists, the heat transfer efficiency is proportional to the surface area of the heated portion of the capillary tube, multiplied by the time of the contact for a given quantitity of fluid. Accordingly, heat transfer efficiency is proportional to the length of the heated portion of the capillary tube and to the third power of the diameter of the capillary tube. A heated capillary tube with a diameter of 50 microns accordingly must have a heated length twenty seven times the heated length of a capillary tube with a 150 micron diameter in order to maintain the same heat transfer efficiency.
Smaller diameter capillaries also cause problems with respect to the pressure drop of the liquid through the vaporizer. For example, the total pressure required to drive 1 ml per minute of sample through a 50 micron diameter capillary in order to obtain near complete vaporization is about 200 bar. This pressure accordingly requires a substantial part of the capacity of the LC pump, so that the range of pressure drop available for the LC column is proportionally less. Also, this significant head pressure through the capillary tube makes the use of in-line auxiliary detectors less desirable.
In view of the above problems, prior art vaporizers have generally tended to utilize uniform diameter capillary tubes with the exit diameter conforming to the diameter of the capillary tube. While such prior art vaporizers provide satisfactory performance for many applications, performance is comparitively poor for vaporizers being provided with low flow rates. A practical technique for providing reliable performance from a thermospray vaporizer being supplied a liquid sample at a low flow rate is thus needed in order to enhance the versatility of the thermospray technique.
The disadvantage of the prior art are overcome by the present invention, and improved thermospray techniques are hereinafter disclosed suitable for interfacing liquid chromatography and mass spectrometry.